Communication network technology is advancing at a rapid rate. For example, all-optical networks using wavelength division multiplexing (WDM) are currently being deployed for a wide variety of communication applications. WDM techniques allow optical signals having different wavelengths to be multiplexed into a single optical fiber. Each of the wavelengths serves as an optical carrier and can be used independently of the other wavelengths, such that different wavelengths may use different modulation formats to carry different signal types. In one possible implementation, each wavelength may carry a modulation signal representing a synchronous optical network/synchronous digital hierarchy (SONET/SDH) client payload, where each client is a SONET-rate time division multiplexed (TDM) application and the common carried signals are in an OC-x format, where “OC” denotes optical carrier and x denotes the rate, e.g., an OC-3 format, an OC-48 format, an OC-192 format, etc.
Such optical networks generally include routing elements such as wavelength switching cross-connects, wavelength adapters, wavelength interchanging cross-connects, etc. A wavelength switching cross-connect serves to cross-connect incoming wavelengths on a given input fiber to different output fibers, but does not provide any transformation in wavelength. When only this type of routing device is present in an optical network, the network typically routes a given end-to-end demand using a single wavelength. If a primary network path assigned to the given demand fails, the demand generally must be carried on a secondary or restoration path using exactly the same wavelength as the primary path. A wavelength adapter is a device which allows conversion of wavelength at the client-network interface. A wavelength interchanging cross-connect is used to cross-connect incoming wavelengths onto different output fibers while also providing transformation of wavelengths.
One type of approach to providing failure protection in an optical network formed of these and other elements is to provide complete redundancy, such that the network includes a dedicated backup or secondary connection for each primary connection of the network. When a link, span or node of the primary connection fails, traffic may then be switched onto the corresponding elements of the secondary connection.
FIG. 1 illustrates a conventional failure protection approach of this type which is particularly designed to protect against nodal failures. The approach is commonly referred to as “dual homing.” The figure shows a portion of a network which includes a source network element NE-A and a destination network element NE-Z. Associated with NE-A is a set 10 of dual-homed network elements 10-1 and 10-2. Associated with NE-Z is a set 12 of dual-homed network elements 12-1 and 12-2. Each of the network elements shown may correspond to a router, a switch or a cross-connect. In the conventional dual-homing approach, a single demand between NE-A and NE-Z is translated into two demands S1 and S2, each requiring the same capacity as the original single demand. The two demands S1 and S2 are routed between NE-A to the set 10 of dual-homed network elements, and similarly from the set 12 of dual-homed network elements to NE-Z. Although this dual-homing approach provides protection against nodal failures, the doubling of demands leads to excess network capacity and hence increased cost.
One particular application of the conventional dual homing approach illustrated in FIG. 1 is in a network which is divided into multiple administrative domains, operational domains or other types of domains. In such an arrangement, traffic is often handed off from one domain to another via a node which operates as a gateway between the two domains. Unfortunately, the above-described conventional dual homing approach is unable to provide adequate and efficient protection in this application as well as other important applications.
What is needed is an improved dual-homing approach which overcomes the above-noted problems associated with the conventional dual-homing approach.